The field of the invention is that of telecommunications. To be more precise, the invention concerns lawful telephone tapping systems, also known as call interception systems.
Telephone call interception systems have been used for many years in lawful telephone tapping systems (i.e. systems under the control of a defined authority, conventionally the state).
The oldest technique consists in applying a physical tap to the cables of a subscriber under surveillance. Unfortunately, this technique is impractical and easily detectable. Moreover, it is not compatible with digital communications (Integrated Services Digital Networkxe2x80x94ISDN) or with mobile telephone systems.
Another technique consists in analyzing everything on a given line and then extracting therefrom the pertinent information. Clearly this solution is costly and complex, and ill-suited to universal telephone tapping, regardless of the communications system used.
A further technique that has been used is to create a conference call bridge within the exchange to which the subscriber to be intercepted is connected. The problem with this technique is that defects due to the various parties involved in the conference call bridge, and in particular the interceptor, are additive. It is therefore relatively easy to detect. Moreover, it is not possible to effect more than one interception of the same subscriber using this technique.
However, it is desirable for this to be possible in some situations. This is the case in particular when the same communications system covers geographical areas administered by different authorities. For example, a mobile radio system satellite can conventionally cover a plurality of states, each of which has its own law and its own requirements in terms of lawful telephone tapping. Consequently, various telephone tapping control centers must be provided (one per area). It must of course be possible for more than one of these centers to intercept the same subscriber, if necessary.
Generally speaking, the various systems that have been used in the past have at least some of the following drawbacks:
they are easily detectable by a subscriber,
they necessitate a great amount of processing to extract the wanted information from the signals intercepted,
they cannot be used interchangeably on all types of network (analog telephone network, digital network, cellular mobile radio network, satellite communication network, etc.),
they do not provide for selective interception as a function of different centers associated with separate geographical areas,
they are not completely effective against call rerouting and subscriber number changing techniques,
they cannot transmit intercepted data to more than one separate control center,
etc.
One objective of the invention is to overcome these various drawbacks of the prior art.
To be more precise, a first objective of the invention is to provide a method of intercepting calls sent and/or received by a subscriber, regardless of the communications systems employed, even if the subscriber uses particular means such as call rerouting, call transfer, number change, etc.
Another objective of the invention is to provide a method of the above kind which can intercept all types of call (voice, fax, data) and all required signaling information.
A further objective of the invention is to provide a method of the above kind which necessitates simple and effective processing of the data to be intercepted and which is totally undetectable by subscribers.
A further objective of the invention is to provide a method of the above kind providing for selective telephone tapping in different geographical areas and for transmitting an intercepted call to a plurality of interception control centers, corresponding to different beneficiaries (the courts, the police, etc.), for example.
A further objective of the invention is to provide a method of the above kind which provides for selective tapping of a group of lines (corresponding to all the lines used for an exhibition or all the lines of a company, for example).
The above objectives, and others that emerge more clearly hereinafter, are achieved by the invention by means of a method of intercepting a telephone call in a telecommunications system using at least one switch, in which method signals in an interception loop in said switch and to which the pulse coded multiplex (PCM) containing the call to be intercepted has been diverted are sampled at high impedance before they are directed to the addressee.
This makes it possible to perform lawful telephone tapping without this being detectable by the users. There is no restriction on the communications systems used (in particular digital and mobiles systems).
The signal sampled at high impedance is advantageously directed to interception equipment which extracts said call to be intercepted from the PCM. The interception equipment can be inside the switch or at a separate location.
In a first embodiment of the invention, the signals in said interception loop comprise voice data and signaling interception data. In this case, the interception equipment first extracts the interception data from the sampled signal and then performs the interception proper.
In a second embodiment, offering enhanced security, the signals in said interception loop comprise only voice data, interception data concerning said call to be intercepted being transmitted to said interception unit on a dedicated line by control means of the switch handling said call to be intercepted.
In this case, knowing only the content of the sampled signal is not sufficient to perform the interception.
In a third embodiment, part of the interception data from a call is transmitted together with the signals conveyed by said interception loop, and part via a dedicated line.
Said interception equipment is advantageously connected to an interception control center which in turn feeds one or more separate telephone tapping centers. In the invention, there is no intrinsic limitation on the number of telephone tapping centers it can feed.
Each of said telephone tapping centers preferably corresponds to a geographical interception area, such as a state or an administrative region. More than one telephone tapping center can of course be allocated to the same geographical area.
Each of said geographical interception areas is defined by at least one of the following, for example:
a list of identifiers of cells of a cellular network forming said geographical interception area,
a list of dialing prefixes allocated to said geographical interception area,
at least one item of information, supplied by way of a signaling parameter, defining the geographical interception area to which the call to be intercepted relates, and
at least one item of information, supplied by the switch, defining the geographical interception area to which the call to be intercepted relates.
This assures great flexibility, the invention requiring no correlation between the geographical interception areas and the coverage areas of the switch.
An advantageous feature of the invention is that said interception can cover a plurality of lines corresponding to the same source and/or the same destination (for example an exhibition, a company, etc.).
Said telecommunications system can be one of the following:
an integrated services digital communication network,
a cellular mobile radio network, or
a satellite radio network.
The invention also concerns the interception equipment used in a method as described hereinabove.